Idea: Define process links in files

[brotkrueml]

Configuration should not be stored in the database, instead files should be used. This has advantages:

• Configuration and its history can be versioned via Git.
• Configuration is deployable.

TYPO3 v12 will probably provide a way to read configuration from files and hydrate value objects accordingly, see discussion:
https://decisions.typo3.org/t/configuration-overhaul-plan/728

Prerequisite: #7

Acceptance criteria:

• Process links are defined in yaml files.
• An upgrade wizard to move the configuration from the database into yaml files is available.
• The backend module displays all available process links readonly.
• The Process, Processtablefield and Step model are removed.
• The ProcessRepository and the StepRepository are removed.
• The field step_uid is removed from the transfer table. Also the logic to use the step_uid as fallback when starting processes is in place.
• The documentation is adjusted.
• Add breaking entry to changelog.

The following fields for a process link are available:

• name (string, unique, required) - corresponds to process name in JobRouter
• connection identifier (string, required)
• processtablefields (array, required)
• steps (array, required)

The following fields for a process table field are available:

• name (string, required) - corresponds to process table field name in JobRouter
• type (string, required)
• field_size (string, optional, defaults to 0)

The following fields for a step are available:

• identifier (string, required, formerly known as "handle")
• step number (int, required)